Single storey extention covering the whole width of house

Your architect should be able to answer your questions for your if not change him, "you may be ok with permitted development" but you will still need build regs approval so the council will know what you are doing Peter

ask your architect which application he wants you to apply for. It may be one of three options: 1 - the recent neighbourhood consultation (informally asking the council if something that doesn't need planning normally is still ok) you will need to submit a basic plan, may need to pay a fee, neighbours are asked for feedback and the council can still ask you to apply for planning. 2- a lawful development certificate (formally obtaining a certificate to legally prove a project does not need planning) you need to pay a fee, submit plans, neighbours are not informed but again still a risk you may be asked to apply for planning. 3 - formal planning permission (project needs planning and you are asking for the council's permission). This is the safest option. I would advise going ahead with whatever route your architect advises. The money you pay the council will be minimal compared to the cost of the extension and it will save a lot of potential headaches and more cost if you get told to stop or potentially amend/remove your extension. Louise UK Property Services

You can submit under Permitted development, but you cannot go higher than 3m. I assume this is why your architect is telling you to submit full planning. If so then he/she is correct and you should take their advice and trust their recommendation. They should have explained this to you though. Hope this helps